The invention relates to circuitry for generating a signal proportional to an impedance to be measured, wherein a two-wire connection is made to the impedance via an active measuring converter having an oscillator for supplying the impedance with a stabilized alternating voltage.
Such measuring systems are known, for example, in German Pat. No. 1,249,404. In addition to an oscillator the active measuring converter comprises a measuring circuit having an emitter coupled transistor, and a capacitor and the impedance to be measured in parallel in the emitter circuit of a coil. A two-wire connection couples the measuring signal to an A.C./D.C. voltage converter connected to a voltmeter. Power is supplied to the measuring connection from a separate D.C. voltage line source. One side of the D.C. voltage source and one measuring point connection are connected to ground.
Very often, when there are great distances between the measuring points and the place for evaluating the measurements, active measuring converters will be located at the objects to be measured. The signals from the measuring converters for each measuring point are transmitted to the place of evaluation via two-conductor lines. On the two-conductor line there can be transmitted both the current for supplying the active measuring converter and the signal current representing the measured physical value. For example, one can supply 4 mA direct current for operating the respective measuring converter while a range of 16 mA may be used for the transmission of the measuring signal. The cable expenses can be reduced considerably. If the active measuring converter converts the measuring signals into direct current signals, no specially adapted signal converters can be supplied by an uncritical D.C. supply. The supply voltage, however, should not fall below a minimum limit of 12 volts while the maximum limit usually must be at 36 volts.
To determine filling height levels in tanks, capacitance differences are measured by sondes which, according to the filling height, are completely, partially or not at all covered. Generally such capacitance devices are subjected to great losses. In addition to the purely capacitive current the oscillator of the measuring converter must therefore meet a loss of current, which depends upon the voltage applied to the sonde. This voltage cannot be chosen as low as one would like because this will result in an unfavorable ratio of useful to interfering signals, impairing the accuracy of the measurement. However, if one increases the output voltage of the oscillator in order to avoid this unfavorable useful/interfering ratio, then an operating current of 4 mA for the oscillator does not suffice.
Difficulties arising if the operating current is too high as compared to the measuring current can be avoided by transmitting the measuring signal as a superposed alternating component on the two-wire line while direct current is supplied to the measuring converter. For an alternating component one can use a signal with variable frequency. The measured quantities can also be digitalized and coupled as coded pulses to the two-wire line. Such transmission methods require expensive coding circuitry in the active measuring converter. Decoding systems are required at the receiving end of the two-wire line. Further, the transmission is prone to interferences, since voltages and currents received from foreign A.C. sources will influence the measured quantities. There will also be interference coming from the measured quantities in the form of alternating components, due to electric and magnetic couplings to adjacent lines. Interfering radiation can also be produced at higher transmission frequencies or pulse transmission frequencies. Thus, one cannot use inexpensive direct current measuring via one of the conductors and a moving-coil indicator.
Measuring converters having waveforms deviating from the sine curve will be radio jamming sources due to their content of strong high-frequency upper harmonics. Therefore in most cases it is not permitted to use them.